It is well-known that a mismatch between the output resistance value of a RF-amplifier and the input resistance value of a load RF-element which is signal-fed by the RF-amplifier produces voltage overshooting. This voltage overshooting occurs at the transmission link or connection between the RF-amplifier and the load RF-element, and can damage the RF-amplifier or cause accelerated aging of this latter. The voltage standing wave ratio, or VSWR, is commonly used for quantifying the voltage overshooting.
Then, sensing and detection circuits have been used for detecting the voltage on the transmission link or connection between the RF-amplifier and the load RF-element, and for adapting parameters of the RF-amplifier or of an intermediate attenuator in order to limit the VSWR value. But such solutions have several drawbacks. First, the sensing circuit causes insertion loss, which results in additional power consumption by the RF-amplifier. Second, it is necessary to sense both the forward and the reverse power transmitted between the RF-amplifier and the load RF-element in order to avoid driving an overprotection which leads otherwise to non-optimized performances. Moreover, this needs to compute the detected signal to make a decision, which is time-consuming. In addition, the control is conducted on parameters effective at the input of the RF-amplifier, such as biasing, input RF-signal, power-amplifier sizing, gain sizing, etc, which also influences other blocks such as drivers, mixers, etc, and this may not be desired.
Another issue is that such RF-amplifier protection does not implement only MOS technology, thus leading to cost increase.
FIG. 1 is a circuit diagram of a known transmission chain including a RF-amplifier and another protection implementation against voltage overshooting. The reference numbers indicated therein are the following ones:                1: a source module which comprises a RF-generator        2: the RF-amplifier which is connected for amplifying a RF-signal produced by the source module        3: the load RF-element, for example a transmission antenna        4: an envelope detection module        5: an active protection module        6: the transmission link for the RF-signal from the RF-amplifier to the load RF-element        
Other references have the meanings now recited:                VBAT: voltage of a power supply of the RF-amplifier 2        M1: power transistor of the RF-amplifier 2        Rb: biasing resistor of the power transistor M1         D: drain terminal of the power transistor M1         GRD: reference terminal of the RF-amplifier 2        Ld, Cd: inductor and capacitor implemented for adjusting the output resistance value of the RF-amplifier 2        S output terminal of the RF-amplifier 2        A: input terminal of the envelope detection module 4        I: intermediate node of the envelope detection module 4        R1, C1: resistance and capacitor of the envelope detection module 4        D1: diode of the envelope detection module 4        L1: inductor of the envelope detection module 4        B input terminal of the load RF-element 3        
The input terminal A of the envelope detection module 4 is connected to a node belonging to the transmission link 6.
Usually, the inductor Ld and the capacitor Cd are selected so that the output resistance value of the RF-amplifier 2 substantially matches the input resistance value of the load RF-element 3 for intermediate voltage values of the RF-signal. But this matching no longer applies when the voltage of the RF-signal becomes important, because the output resistance value of the RF-amplifier 2 decreases for high RF-voltage values. For example, intermediate RF-voltage values denote RF-voltage values which are less than 1.2 the supply voltage value VBAT, and high RF-voltage values denote RF-voltage values higher than this limit.
The RF-frequency of the RF-signal which is produced by the source module 1 may be 2.5 GHz (gigahertz). The respective values of the resistance R1 and capacitor C1 may be respectively 10 kΩ (kilo-ohm) and 10 pF (picofarad) for instance. The value of the inductor L1 may be 10 or 15 nH (nanohenry). If the supply voltage VBAT of the RF-amplifier 2 is positive with respect to the reference terminal GRD, then the input terminal A of the envelope detection module 4 is connected to the anode of the diode D1, and the cathode of the diode D1 is connected to the intermediate node I. If the supply voltage VBAT is negative with respect to the reference terminal GRD, then the input terminal A is connected to the cathode of the diode D1, and the anode of this latter is connected to the intermediate node I.
The operation of the envelope detection module 4 is well known: the parallel-connected resistance R1 and capacitor C1 act as an averaging circuit which can be fed by the diode D1 at each positive swing of the RF-voltage existing at the input terminal A. The inductor L1 is connected between the input terminal A of the envelope detection module 4 and the reference terminal GRD of the RF-amplifier 2. It sets the diode D1 in blocked state for the direct voltage component (DC-voltage), but it is not effective to the RF-signal. Thus, the voltage at the intermediate node I is an averaging of the maximum swing values of the RF-signal.
The active protection module 5 is fed at input with a signal produced from the envelope detection module 4, and acts on parameters of the source module 1 and/or the RF-amplifier 2 when the envelope reaches high values. The signal fed into the active protection module 5 may be directly the voltage VI at the intermediate node I, corresponding to the average value of the voltage magnitude of the RF-signal which exists on the transmission link 6. But such protection implementation requires using the active protection module 5, and thus has some of the drawbacks listed before.
Therefore, an object of the present invention is to provide an efficient protection for a RF-amplifier against voltage overshooting, which does not have these drawbacks. In particular, the present invention aims at protecting the RF-amplifier without requiring that its output voltage is monitored continuously.
Another object of the invention is to provide a protection which is efficient when the output RF-voltage becomes important and makes the output resistance value of the RF-amplifier to decrease appropriately.
Still another object of the invention is to provide a protection which is simple to implement and low cost.
Still another object of the present invention is to provide a protection which may be easily adapted to varying operation conditions of the RF-amplifier, in particular when a power supply voltage of the RF-amplifier varies or is changed voluntarily.
Still another object of the present invention is to provide a protection which may be easily adapted to actual toughness and parameters of the RF-amplifier, in particular the power transistor of this latter.